Case-hardening process for steel articles



Patented July 12, 1927.

UNITED STATES F Fl C E CASE-HARDENING P ROCESS FOR STEEL ARTICLES.

No Drawing. Application filed November 21, 1925,

In order to impart a surface as hard as glass to certain parts of steel articles, whilst leaving the core in a con'iparativcly soft condition, the process known as case hardening r is employed. Inthis process, the surface of the articles is brought into intimate contact, either all over or only at selected portions where the glass-hard surface is to be produced, with substances which furnish l carbon, and .the articles are then heated to redness for a suitable time, in the course of which carbon is absorbed by the superficial layer-sand increases their carbon contents. The articles are then heated again, to hardening temperature, and then quenched in oil, water or the like, to effect the hardening. The steels generally employed for ordinary case hardening are low in carbon (O.12O.25%C.) that is to say, mild steel, in which the influence of the hardening treatment is comparatively slight in the portions which have not been artificially enriched with carbon.

Recently, however, there has been a con stantly increasing demand for the production of certain machine parts from steels of higher mechanical properties, and with surfaces, or parts of the surface, as hard as glass, so as to increase the strength of these duced. Such high-tensile steels always exhibit, in their composition, relatively high percentages of carbon, or other hardening constituents, and if process were employed, not only would the outer layers acquire the hardness of glass, but the mechanical properties of the uncarburized portions of the article would be modified. The limit of tensile strength in particular would be raised, but at the expense of the elasticity and toughness of the material, which would become harder throughout, but at the same time more fragile or brittle. y

In order to retain a suificient degree of toughness in the main mass of the article after hardening such parts when made of high tensile steels, it is usual to employ, in such cases, not the ordinary carbon steels, but alloyed steels, and particularly nickel steels, since the result of adding nickel is to preserve the toughness in the interior of the material, to a certain degree, even after quenching.

The additin of nickel considerably .increases the cost of the material and ofthe parts and enable their dimensions to be rethe above described- Serial No. 70,675, and in Germany November 17, 1924.

articles made therefrom, and has the further drawback of lowering the resistance of the carburized layers to frictional wear. In order to obviate the necessity for adding nickel, and still enable high tensi'le steels to be provided with glasshard outer layers over the whole or parts of their surfaces, without addition of nickel, and without impairing the elasticity and tou hness of the interior of the steel, the following procedure is adopted, according to the invention.

The enrichment in carbon on those parts of the surface which are afterwards to be made as hard as glass, is effected in the usual manner. This is preferably followed by annealing or tempering the entire article. for the purpose of conferring the desired qualities thereon. In order to impart the hardness of glass to the surface layers which have been carburized in the usual way, and eitherafter the said intermediate treatment or normalizing, or without such treatment, the method adopted difiers from that previously known and practised, in that the steel body, only in the external layer forming the zone enriched with carbon or containing the artificially carburized portions, is heated to the requisite throughout the depth to which the artificially supplied carbon has penetrated or to which the glass-hardness is desired to be produced in the carburized portions of the surfaces.

The heating to hardening temperature is therefore not intended to extend to the in nermost portion of the article, and therefore the core material retains its original high ualities, or those imparted by normalizing iardening and tempering or annealing. The superficial heating, which in no case should raise the steel to hardening temperature for more than a few millimetres in depth, and then rapidly diminish towards the interior, must be effected so quickly that the heat supplied, whilst sufficient in degree and quantity, has no time to communicate itself to the interior core. With'this object, it is advisable to effect, the heating in a highly heated bath of lead or other metal or alloy containing a suflicient amount of heat, in relation to the dimensions of the article under treatment, to prevent the temperature of the bath from being reduced below hardening temperature through the absorption of heat by such article.

In order to prevent the occurrence of hardening temperature though presenting high resistance to stresses in the interior of the article it is advisable to heat the whole of the article, prior to its immersion in the highly heated bath, to atemperature below the limit at which the heat would begin to have any modifying effect on the structure of the material.

The heating of the surface to hardening temperature in the heating bath is effected in a very short time, whereupon the article is quickly taken out of the bath and rapidly quenched in the hardening liquid.

This process does not presuppose the presence of any nickel in the steel, and enables such nickel to be dispensed with. On the other hand, it is-equally applicable to nickel steels and non-nickel steels.

The glass-hard carburized layer, extending. over the'whole or parts of the surface of case hardened steel articles, need only be comparatively thin, because, as the result of the extreme hardness imparted by quenching after heating, this layer is either not intended to, and does not, wear away. or it it does, the extent of the wear must be very small since only a narrow limit of alteration in dimensions through wear is permisf lllgle in the case of machine parts or the If, as has generally been the case hitherto, the steel of thearticle to be case hardened consists of mild steel, the glass-hard layer must, nevertheless, be of a certain thickness, because, owing to the comparative softness of the underlying mild steel core, this layer,

abrasion, is liable to bend or buckle inwardly.

The same danger exists when the core is r not of mild steel, but of steel with higher properties, which has been treated by the process according to the invention.

' The new case hardening process, however, also atfords the possibility on the one hand, of restricting the glass-hard layer and the requisitecarburization of said layer from the natural condition of the steel, to the extent corresponding to the requirements of the case; and on the other hand enables the production of depressions or other changes in the shape of the glass-hard layer, through external influences, tobe prevented.

According to the process outlined above, the heating'of the steel parts may be carried out in such a way that the heat requisite for the desired portion of the article is only imparted to the ature as will which consists "lass hardening of the carburized depth of such portion, from which point onward it should diminish through a narrow zone to the temperature of the relatively cool core or to the degree to which the whole piece was preheated. The newprocess may also be conducted in such a way that the heating to hardening temperature will not be restricted to the limit of the carburized layer, but will extend inwardly a short distance beyond, in consequence of which the subsequent quenching will produce a hardened Zone of uncarburized material between the carburized glass-hard layer and the core. This zone may be of any desired thickness and forms the transition stage to the core steel which has not been affected by the hardening treatment.

This second zone, of hardened but uncarburized material, exhibits, in its layers adjacent to the carburizer layer, the degree of hardness imparted to the same class of steel by ordinary hardening, whilst the out side layer covering or surrounding same eX- hibits. the glass hardness due to the previously artificially introduced carbon. The core, which was not heated to hardening temperature prior to quenching, retains the mechanical properties inherent in the material or imparted thereto by previous temporing.

What I claim is 1. A process for treating a steel article which consists in carburizing external portions of such article, then heating the article in such a manner that the portions underlyingthe carburized portions are not heated to hardening temperature, while the carburized portions are heated to such a temperproduce a glass-hard condition upon subsequent quenching from and then quenching the therein such temperature, article.

2. A process for treating a steel article in carburizlng an external layer of such article, heating the article throughout to a ten'iperature which will reliere internal stresses but which is below hardening temperature, then heating said layer to such a temperature as will produce a glass-hard condition therein upon subsequent quenching from such temperature while not substantially raising the temperature of the remainder of the article, and then quenching the article In testimony whereof I have afi'ixed my signature.

FRANZ PACHER. 

